Atmospheric pressure

• weight of atmosphere above you.  So it decreases as you go up in altitude.
• Rule of thumb for change with altitude is 10 mb per 100 meters.
• More accurate rule of thumb would be Pressure = 1013mb x exp(-Altitude/8.5), where Altitude is the altitude above sea level
• mathematically defined as force/area.  In this case the force is the weight of the molecules, which happens to be about 14.7 lbs/square inch at the surface at the Earth.
• Typical value at sea level is ~ 1013 mb.
• Typical station pressure value for Tucson is ~ 925 mb.
• Variation of about 5 mb over the course of a day.
• Worldwide the lowest sea level pressure has been 870 mb (in the eye of Typhoon Tip) and the highest has been 1083.8 mb (in a region of northern Russia).
• Pressure Gradient
• Defined as the change in pressure between 2 points divided by the distance between the 2 points.

  Pressure Gradient = (Pressure change)/(distance)
• It is directed from higher pressure toward lower pressure.
• This applys in both the vertical direction and the horizontal direction.
• The vertical pressure gradient is larger than the horizontal pressure gradient.